The purpose of the studies proposed here is to elucidate the origin of transformation defective viruses in the Avian tumor virus system and to attempt to answer the question of why frequency recombination within the src gene does not occur in crosses involving the Bryan strain of Rous sarcoma virus. The first goal will be pursued by analyzing the progeny virus from cells known to possess either an endogeous or an exogenously derived provirus and to compare the proportion of td virus in these stocks with that of virus propagated in cells not known to possess either an endogenous or an exogenous provirus. Crosses between different ts mutants of the Bryan strain RSV will be carried out to check if recombination occurs in these homologous crosses. The early interactions of cells and viruses (adsorption and penetration) using ASV and chick embryo fibroblasts of known resistance or susceptibility to ASV will be investigated with the sensitive technique of laser flow cytometry to gather information on the presence and nature of specific and non-specific receptors on the cell membrane.